1. Field of the Invention
This invention relates to a thermionic cathode for an electron beam device, and more particularly to a thermionic cathode for an electron beam device comprising a layer of thermoelectron emissive compound, a barrier layer and a layer of metal of high melting point.
2. Prior Art
In recent years there has been a need for electron beam of high intensity because of two purposes. One purpose is to utilize such electron beam for analyzers such as transmission electron microscope, scanning electron microscope or Auger spectroscopic analyzer in which electron beam is utilized. The other purpose is for use as an electron gun for electron beam rethography which is used instead of photorethography.
It is known from U.S. Pat. No. 2,639,399 that metal borides such as CaB.sub.6, SrB.sub.6, BaB.sub.6, LaB.sub.6, CeB.sub.6 or ThB.sub.6, particularly rare earth metal borides have small work function, thus exhibiting good thermoelectron emissive properties for use as cathodes in various electron beam devices. However, with most borides, particularly lanthanum hexaboride, it has been difficult to operate in stable conditions for a long time under elevated temperature required for the operation of electron beam devices since borides react with metal supporting the borides thereon.
Taking such reaction into consideration, a method has been proposed in U.S. Pat. No. 2,589,104 in which a tip of lanthanum hexaboride is supported on a cooled metal support and heated by a tungsten coil disposed around but spaced from said tip. Alternatively, in U.S. Pat. No. 2,807,675 there has been disclosed a structure having a reaction barrier material between an emitter tip of lanthanum hexaboride and a support metal, and provided with thermally decomposable graphite serving as a heater. However, the former device has a complicated structure and furthermore requires a large electric power upon operation, say 20 to 60 W because it utilizes indirect heating system, while in the latter device the thermally decomposable graphite deteriorates or softens at operation temperatures, hence not only resulting in unstable beam but also calling for large electric power. Therefore, notwithstanding these cathodes emitting electrons with high intensity, it has been difficult to apply such cathodes instead of hairpin type tungsten cathodes to electron beam analyzers.